This invention relates to a hermetic package for hybrid integrated circuits and to a method for making a package. More specifically, this invention relates to a package which has the flexibility to be used for a variety of integrated circuit functions.
A hybrid integrated circuit is a circuit built up from individual components in die form which may include one or more monolithic integrated circuits and one or more discrete devices. The individual integrated circuit and discrete devices, in die form, are usually mounted and interconnected in a sealed package enclosure which protects the die from physical damage and from the ambient. The package also provides external leads for electrically contacting the integrated circuit and connecting it in its intended use.
In general, the conventional hybrid integrated circuit ceramic package has a flat ceramic bottom with a ceramic seal ring attached to form the bottom and side walls of the package enclosure. The seal ring has metallized feedthroughs to provide electrical contact between the package interior and the outside world. The inner portion of these feedthroughs terminate in bonding pad areas; external leads are brazed to the outer portion of the feedthroughs to facilitate connecting the circuit to a printed circuit board or other application. The package bottom is provided with a metallized substrate attachment pad which is simply a rectangle of solderable metal screen printed or otherwise applied to the ceramic bottom. As so far described, the package is standardized and can be used for almost any circuit function. To be used for a particular circuit function, however, a patterned substrate must first be soldered or brazed to the substrate attachment pad. The patterned substrate is unique to each circuit function. It has on its surface a metallized pattern which provides die attach areas, wire bond pads, and lines for electrical interconnection. The circuit components, which may consist of monolithic integrated circuits or discrete device chips, are die bonded to the die attach areas. The chips are appropriately interconnected by wire bonds and the metallization provided. Electrical contact between the completed circuit within the package and the outside world is accomplished by wire bonding from the patterned substrate to the feedthrough bonding pad areas. The circuit function on the substrate is thus connected to the external leads via wire bonds and the feedthroughs provided on the seal ring. The hermetic package is completed by soldering a metal or ceramic lid to the top of the seal ring. This package has the disadvantage that a separate substrate must be used for circuit function. Also, the additional step of attaching the substrate to the package bottom is required. Most importantly, however, wire bonds are required to complete the connection between the substrate and the external leads. The use of these additional wire bonds is expensive, time consuming, and detracts from the reliability of the circuit.
An alternate packaging technique for hybrid integrated circuits is to provide a metallized pattern directly on the bottom ceramic. This patterned bottom can take the place of the metallized and patterned substrate and can also provide the feedthrough function. The appropriate parts of the interconnect metallization pattern can be routed to the edge of the ceramic bottom where external leads can subsequently be attached by brazing. Wire bonds from the substrate to the feedthroughs are thus eliminating. The difficulty with this technique, however, is that the ceramic seal ring must be attached to the package bottom by firing at a high temperature. This is done by applying a layer or glass between the two ceramic pieces and then raising the temperature to about 900.degree. C. or higher depending on the glass used. The choice of metals that can be applied to the package bottom is thus limited to those metals that would not be adversely affected by this high firing temperature. Some metals such as gold, which are very desirable for reasons of solderability, bondability, current carrying capability, and resistance to corrosion, cannot be used because of diffusion problems between the gold and underlying metals. Also, because the package bottom must be customized for each circuit, a different package is required for each circuit function. Hybrid integrated circuits are usually designed for very special applications. As such, the number of any particular circuit is relatively small so supplying a different package for each circuit function can be very expensive.
Accordingly a need existed to develop a hybrid integrated circuit package, including a method therefore, which would overcome the problems of prior art packages and methods to produce a reliable, low cost, flexible package.
It is an object of this invention to provide an improved hybrid integrated circuit package. The package has the flexibility to be adapted to a number of different circuit functions. A separate metallized substrate is not required and no wire bonds are required to interconnect the completed circuit function to the external leads.
It is a further object of the invention to provide a process for fabricating an improved hydrid integrated circuit package. The process permits the patterning of the package metallization to accommodate a variety of circuit functions.